Gas sensor

ABSTRACT

A protective cover of a gas sensor includes an inner protective cover which covers at least an end portion of a sensor element, an outer protective cover which covers the inner protective cover, and an intermediate protective cover which is installed between the inner protective cover and the outer protective cover. A 1 /A 2 ≧1 provided that A 1  represents a total opening area of the inner gas inlet holes provided for the inner protective cover, and A 2  represents a total opening area of the outer gas inlet holes provided for the outer protective cover.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a gas sensor for measuring gascomponents such as NO, NO₂, SO₂, CO₂, and H₂O, contained in theatmospheric air or the exhaust gas discharged from vehicles orautomobiles, for example. In particular, the present invention relatesto a gas sensor which has a protective cover surrounding a sensorelement.

2. Description of the Related Art:

A variety of gas sensors have been suggested and practically used. Forexample, there are oxygen sensors based on the use of oxygen ionconductors, NOx sensors (see Japanese Laid-Open Patent Publication No.8-271476), HC sensors (see Japanese Laid-Open Patent Publication No.8-247995), hydrogen sensors based on the use of proton ion conductors,H₂O sensors, oxygen sensors based on the use of oxide semiconductorssuch as SnO₂ and TiO₂.

Among the gas sensors as described above, the oxygen sensor based on theuse of ZrO₂ and the oxygen sensor based on the use of TiO₂ are widelyused for controlling the oxygen concentration in the exhaust gasdischarged from an automobile and/or controlling A/F (air-fuel ratio),because such gas sensors operate stably even in the environment of theexhaust gas discharged from an automobile. The NOx sensor, which isbased on the use of ZrO₂, is also at the stage of practical use tocontrol NOx discharged from an automobile.

As an oxygen sensor attached to an exhaust tube of an internalcombustion engine, a sensor having a protective cover provided around asensor element to bring about a uniform flow of the exhaust gas, or asensor having a protective cover for avoiding condensed water producedupon the start-up of the engine (droplets of water) is used. Sensorsdescribed in U.S. Pat. Nos. 4,597,850 and 4,683,049 are known as oxygensensors to each of which a protective cover of a double structure isattached.

As for the conventional protective covers as described above, if a waterprotective cover is used, the response of the gas sensor may be delayed.Accordingly, it has been suggested that an inner protective cover of adouble-structure protective cover, which is disposed close to a sensorelement, has inner gas inlet holes facing the sensor element forimproving the response performance (Japanese Patent No. 2641346).

However, in the conventional protective cover, the gas sensor issupposed to be positioned upstream with respect to the catalyst. If thesensor and the cover are installed downstream with respect to thecatalyst, a problem was found in relation to water resistance(performance to avoid condensed water produced upon the start-up of theengine). Further, another problem was found in relation to waterresistance depending on the angle of attachment to the exhaust tube ofthe internal combustion engine.

In order to obtain the quick response performance, it is conceived toprovide a structure in which the inflow amount of the measurement gas isincreased. However, in this structure, the droplets of the condensedwater produced upon the start-up of the engine also tend to come intothe sensor element. That is, it is difficult to balance preventing thesensor element from the droplets of water with improving the responseperformance.

Further, considerable temperature change or fluctuation may occur in thesensor element as the measurement gas flows into the protective cover,and cracks may appear in the substrate of the sensor element.

SUMMARY OF THE INVENTION

The present invention has been made taking the foregoing problems intoconsideration, and an object thereof is to provide a gas sensor whichmakes it possible to effectively protect a sensor element from thedroplets of water and reduce the temperature change of a sensor elementcaused by the inflow of the measurement gas without deteriorating theresponse performance and which is excellent in temperaturecharacteristics and water resistance.

According to the present invention, there is provided a gas sensorcomprising a sensor element which measures a predetermined gas componentof an introduced measurement gas, and a protective cover which surroundsthe sensor element, the protective cover including an inner protectivecover which covers at least an end portion of the sensor element, anouter protective cover which covers the inner protective cover, and anintermediate protective cover which is installed between the innerprotective cover and the outer protective cover, wherein the innerprotective cover has a bottom-equipped cylindrical shape with aplurality of inner gas inlet holes which are formed at positions of aside surface thereof facing the sensor element and with at least oneinner gas discharge hole which is formed at a bottom portion; the outerprotective cover has a bottom-equipped cylindrical shape with aplurality of outer gas inlet holes, the outer gas inlet holes formed ina side surface of the outer protective cover at portions where the outergas inlet holes do not face the inner gas inlet holes; the intermediateprotective cover has at least intermediate gas inlet holes which areformed at positions where the intermediate gas inlet holes do not facethe inner gas inlet holes and the outer gas inlet holes; and A1/A2≧1provided that A1 represents a total opening area of the inner gas inletholes, and A2 represents a total opening area of the outer gas inletholes.

Accordingly, the triple structure is provided, in which the intermediateprotective cover is provided for the cover of the double structurecomprising the inner protective cover and the outer protective cover.Therefore, it is possible to effectively avoid the droplets of waterproduced upon the start-up of the engine.

Further, the ratio A1/A2 between the total opening area A1 of theplurality of inner gas inlet holes and the total opening area A2 of theplurality of outer gas inlet holes is not less than 1. Therefore, theflow rate, at which the measurement gas flows from the outer gas inletholes passes through the inner gas inlet holes, is reduced. Therefore,for example, even if the droplets of the condensed water enters theinterior of the outer protective cover through the outer gas inletholes, the droplets of the condensed water does not come to the sensorelement through the inner gas inlet holes, because of the low flow rateof the measurement gas to flow into the inner gas inlet holes. As aresult, for example, even when the protective cover is installed at anyangle with respect to a gas tube (for example, an exhaust tube of aninternal combustion engine), it is possible to avoid the droplets ofwater which would otherwise reach the sensor element. Of course, themeasurement gas does not blow toward the sensor element fast. Therefore,it is possible to suppress the temperature change or fluctuation of thesensor element (temperature change or fluctuation caused by theintroduction of the measurement gas).

Therefore, the protective cover may be attached approximatelyperpendicularly to the gas tube. Alternatively, the protective cover maybe attached while being inclined to the gas tube. It is possible torealize a variety of forms of attachment to the gas tube in conformitywith the preference of the user.

The response performance of the sensor element may be deteriorated dueto the low flow rate of the measurement gas to the inner gas inletholes. However, the deterioration of the response performance can besuppressed by appropriately selecting the diameters of the outer gasinlet holes and the inner gas inlet holes.

In the gas sensor structured as described above, it is also preferablethat the number of the inner gas inlet holes may be larger than thenumber of the outer gas inlet holes. This arrangement decreases the flowrate of the measurement gas in the outer protective cover flowing intothe inner gas inlet holes. Therefore, the measurement gas is diffused inthe inner protective cover until arrival at the sensor element. It ispossible to avoid any local and concentrated emission of the measurementgas to the sensor element.

Therefore, it is possible to avoid local temperature change in thesensor element, and it is possible to effectively avoid, for example,cracks which would be otherwise caused by the temperature change of thesensor element.

In the gas sensor described above, it is also preferable that the innerprotective cover has plate sections each of which extends over each ofinner gas inlet holes. Accordingly, the measurement gas coming into theinner protective cover through the inner gas inlet holes is diffused bythe plate sections. That is, the sensor element is prevented from anydirect blow of the measurement gas. It is possible to suppress thetemperature change in the sensor element.

In the gas sensor constructed as described above, it is also preferablethat the plurality of inner gas inlet holes are formed at approximatelyequal pitches along one circumference of the inner protective cover.Alternatively, when the plurality of inner gas inlet holes areclassified into n groups, then the inner gas inlet holes in a firstgroup are formed at approximately equal pitches along a firstcircumference of the inner protective cover, the inner gas inlet holesin a second group are formed at approximately equal pitches along asecond circumference of the inner protective cover, . . . and similarlythe inner gas inlet holes in an nth group are formed at approximatelyequal pitches along an nth circumference of the inner protective cover.

Usually, if the inner gas inlet holes are randomly formed, themeasurement gas flows intensively toward the sensor element through oneor two of the inner gas inlet holes depending on the arrangement statethereof.

However, when the plurality of inner gas inlet holes are formed alongone circumference or a plurality of circumferences of the innerprotective cover as described above, the measurement gas comes to thesensor element while being dispersed approximately uniformly withrespect to the inner gas inlet holes. Therefore, it is possible tofurther reduce the flow rate of the measurement gas at each of the innergas inlet holes, and it is possible to effectively protect the sensorelement from the droplets of water. It is also possible to avoidintensive blow of the measurement gas against the sensor element.Therefore, it is possible to further suppress the temperature change inthe sensor element.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating, with partial omission, a gassensor according to a first embodiment;

FIG. 2 illustrates a measuring method carried out in a first exemplaryexperiment;

FIG. 3 shows a measuring timing used in relation to the first exemplaryexperiment;

FIG. 4 illustrates standard attachment of the gas sensor;

FIG. 5 illustrates inclined attachment of the gas sensor;

FIG. 6 shows a table illustrating results of measurement as to whethersweat is on the sensor element;

FIG. 7 shows a graph illustrating results of measurement of thetemperature change of a sensor element;

FIG. 8 illustrates the response time brought about when the fuel of anengine is changed from rich to lean;

FIG. 9 illustrates the response time brought about when the fuel of theengine is changed from lean to rich;

FIG. 10 illustrates attachment states of gas sensors for measuring thedelay time;

FIG. 11 illustrates the delay time brought about when the fuel of theengine is changed from rich to lean;

FIG. 12 illustrates the delay time brought about when the fuel of theengine is changed from lean to rich;

FIG. 13 shows a graph illustrating results of measurement of theresponse time with respect to the change of A/F;

FIG. 14 shows a graph illustrating results of measurement of the delaytime with respect to the change of A/F;

FIG. 15 illustrates the response time obtained when the NOxconcentration is changed from 33% to 66%;

FIG. 16 illustrates the response time obtained when the NOxconcentration is changed from 66% to 33%;

FIG. 17 shows a graph illustrating results of measurement of theresponse time with respect to the change of the NOx concentration;

FIG. 18 is a sectional view illustrating, with partial omission, a gassensor according to a second embodiment; and

FIG. 19 is a perspective view illustrating a plate section extendingover an inner gas-inlet hole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Illustrative embodiments of the gas sensor according to the presentinvention will be explained below with reference to FIGS. 1 to 19.

As shown in FIG. 1, a gas sensor 10A according to a first embodimentcomprises a sensor element 12 which measures a predetermined gascomponent, for example, NOx contained in a measurement gas (exhaustgas), and a protective cover 14 which is arranged to surround an endportion of the sensor element 12.

The sensor element 12 is constructed in the same manner as a sensorelement described, for example, in Japanese Laid-Open Patent PublicationNo. 2000-304719 (see FIGS. 3 and 4 of this patent document). Therefore,the explanation thereof will be herein omitted.

In the gas sensor 10A according to the first embodiment, the protectivecover 14 surrounding the sensor element 12 comprises an inner protectivecover 16 which covers the end portion of the sensor element 12, an outerprotective cover 18 which covers the inner protective cover 16, and anintermediate protective cover 20 which is installed between the innerprotective cover 16 and the outer protective cover 18.

The inner protective cover 16 is formed of metal and has abottom-equipped cylindrical shape. The inner protective cover 16 has aplurality of inner gas inlet holes 22 which are formed at positionsfacing the sensor element 12. The inner protective cover 16 also has aninner gas discharge hole 24 which is formed at the bottom (an endportion).

The intermediate protective cover 20 is made of metal and has abottom-equipped cylindrical shape. The end portion of the innerprotective cover 16 is covered with the end portion of the intermediateprotective cover 20. An intermediate gas discharge hole 26 is formed atthe center of the end portion of the intermediate protective cover 20and has a diameter smaller than the diameter of the inner gas dischargehole 24 of the inner protective cover 16,.

The intermediate protective cover 20 has a flange section 28 at a rearportion that abuts against the inner wall of the outer protective cover18. The flange section 28 is integrally formed with a bent section 30.The bent section 30 has a rear end portion which is bent laterally andhas a circumferential end which is bent backwardly. The outercircumferential surface of the bent section 30 abuts against the innerwall of the outer protective cover 18.

The flange section 28 has a plurality of slits 32 which are formed alongthe circumference to define intermediate gas inlet holes. In the firstembodiment, the six slits 32 are formed and equally arranged. Each ofthe slits 32 has a circular arc-shaped form with a central angle of 40°on the circumference of the flange section 28. There are circulararc-shaped intervals between the adjoining slits 32, and each of theintervals has a central angle of 20°.

The outer protective cover 18 is formed of metal and has abottom-equipped cylindrical shape. The outer protective cover 18 hasouter gas inlet holes 34. The outer gas inlet holes 34 are arranged in aside circumferential surface at portions where the outer gas inlet holes34 do not face the inner gas inlet holes 22 of the inner protectivecover 16.

The six outer gas inlet holes 34 are formed and arranged equally in theside circumferential surface of the outer protective cover 18 betweenthe inner gas discharge hole 24 and the bottom of the outer protectivecover 18.

The twelve inner gas inlet holes 22 are formed through a sidecircumferential surface of the inner protective cover 16 such that theholes 22 are provided in two vertically separated positions.

The positions at which the twelve inner gas inlet holes 22 are formedare explained below. Six inner gas inlet holes 22 are formed along onecircumference disposed at a distance L1 from the flange section 28.Further, the other six inner gas inlet holes 22 are formed along onecircumference disposed at a distance L2 (>L1) from the flange section28. In order to maintain the rigidity of the inner protective cover 16,the positions where the respective inner gas inlet holes 22 are formedare set so that line segments connecting the centers of the respectiveinner gas inlet holes 22 make a polygonal curve, so called a zigzagline.

That is, the positional relationship of the outer gas inlet holes 34,the slits (intermediate gas inlet holes) 32, and the inner gas inletholes 22 is established such that the outer gas inlet holes 34, theinner gas inlet holes 22, and the intermediate gas inlet holes 32 arearranged in this order from the bottom to an upper part of the outerprotective cover 18.

The measurement gas coming through the outer gas inlet holes 34 of theouter protective cover 18 arrives at the sensor element 12 via the slits32 of the intermediate protective cover 20 and the inner gas inlet holes22 of the inner protective cover 16. After that, the measurement gas isdischarged through the inner gas discharge hole 24 formed at the bottomof the inner protective cover 16, then through the intermediate gasdischarge hole 26 formed at the bottom of the intermediate protectivecover 20, and finally through the outer gas inlet holes 34 of the outerprotective cover 18. It is assumed that this route is defined as theflow path for the measurement gas. Because the negative pressure iscreated in the vicinity of the intermediate gas discharge hole 26, themeasurement gas smoothly flows through the flow path.

In the gas sensor 10A according to the first embodiment,A 1/A 2≧1provided that Al represents the total opening area of the twelve innergas inlet holes 22, and A2 represents the total opening area of the sixouter gas inlet holes 34.

Accordingly, the intermediate protective cover 20 is provided for thecover of a double structure comprising the inner protective cover 16 andthe outer protective cover 18, thereby providing a triple structure.Therefore, it is possible to effectively avoid the droplets of condensedwater produced upon the start-up of the engine.

Further, the ratio A1/A2 between the opening areas A1 and A2 is not lessthan 1. Therefore, the flow rate at which the measurement gas comes fromthe outer gas inlet holes 34 passes through the inner gas inlet holes 22is reduced.

Therefore, for example, even if the droplets of the condensed waterenters the outer protective cover 18 through the outer gas inlet holes34, the droplets of the condensed water does not come to the sensorelement 12 through the inner gas inlet holes 22, because of the low flowrate of the measurement gas passing through the inner gas inlet holes22.

As a result, for example, even if the protective cover 14 may beinstalled at any angle with respect to a gas tube (for example, anexhaust tube of an internal combustion engine), it is possible toprevent the sensor element 12 from the droplets of water. Of course, themeasurement gas does not collide with the sensor element quickly.Therefore, it is possible to suppress the temperature change of thesensor element 12, such as changes caused by the inflow of themeasurement gas.

Therefore, the protective cover 14 may be attached approximatelyperpendicularly with respect to the gas tube. Alternatively, theprotective cover 14 may be attached while being inclined with respect tothe gas tube. It is possible to realize various options in attachment tothe gas tube in conformity with the preference of a user.

On the other hand, the response performance of the sensor element 12 maybe deteriorated due to the low flow rate of the measurement gas into theinner gas inlet holes 22. However, the deterioration of the responseperformance can be minimized by appropriately selecting the respectivediameters of the outer gas inlet holes 34 and the respective diametersof the inner gas inlet holes 22.

Further, in the first embodiment, the number of the inner gas inletholes 22 is larger than the number of the outer gas inlet holes 34.Thus, the flow rate of the measurement gas into the outer protectivecover 18 is decreased at the respective inner gas inlet holes 22.Therefore, the measurement gas is diffused in the inner protective cover16 until arrival at the sensor element 12. It is possible to avoid anylocal and concentrated emission of the measurement gas to the sensorelement 12.

As a result, it is possible to avoid the local temperature change in thesensor element 12, and it is possible to effectively avoid, for example,the appearance of a crack which would be otherwise caused by thetemperature change of the sensor element 12.

Specific dimensions of the protective cover 14 may be described asfollows by way of example. As for the outer protective cover 18, theouter diameter Do of the outer protective cover 18 is about 14.6 mm, thediameter d1 of the outer gas inlet holes 34 is about 2 mm, and thethickness of the outer protective cover 18 is about 0.4 mm.

As for the inner protective cover 16, the outer diameter Di is about 7mm, the diameter d2 of the inner gas inlet holes 22 is about 1.5 mm, thediameter d3 of the inner gas discharge hole 24 is about 5.0 mm, and thethickness of the inner protective cover 16 is about 0.3 mm.

The diameter d4 of the intermediate gas discharge hole 26 provided atthe bottom of the intermediate protective cover 20 is about 1 mm.

The distance L1 from the flange section 28 to the centers of the innergas inlet holes 22 of a first group is about 3.5 mm. The distance L2from the flange section 28 to the centers of the inner gas inlet holes22 of a second group is about 5.5 mm. The distance L3 from the end ofthe outer protective cover 18 to the centers of the outer gas inletholes 34 is about 2.8 mm. The distance L4 from the centers of the outergas inlet holes 34 to the flange section 28 is about 16 mm. The distanceL5 from the end of the outer protective cover 18 to the end of theintermediate protective cover 20 is about 4.5 mm. The distance L6 fromthe end of the outer protective cover 18 to the end of the innerprotective cover 16 is about 6.5 mm.

Three exemplary experiments (first to third exemplary experiments) willnow be described. In the first exemplary experiment, the sweat and thetemperature change of the sensor element 12 are observed in relation toComparative Example 1 and Examples 1 to 3.

As shown in FIG. 2, the gas sensor (indicated by reference numeral 10)was attached to a downstream portion with respect to a catalyst 56, ofan exhaust tube 54 of an automobile 52 which carried a 2.0-litergasoline engine 50. Further, the rear portion of the automobile 52 waslifted to incline the automobile 52 by 30° with respect to the ground58.

As shown in FIG. 3, the water of 100 cc, which was colored with Japaneseblack ink or the like, was poured into a portion 62 of the exhaust tube54 of the automobile 52 approximately corresponding to a back wheel 60at a time point t0. The application of electric power to a heater of thegas sensor 10 was started at a time point t1 after several seconds to 10seconds. The engine 50 was started (number of revolutions in the idlestate=600 rpm) at a time point t2 after 60 seconds. The accelerationoperation of 3 seconds (number of revolutions at the peak of theacceleration state=5,000 rpm) was performed continuously three times ata time point t3 after 15 seconds.

The judgment whether the droplets of water were on the sensor element 12was made visually.

The temperature change of the sensor element 12 was measured as follows.In order to maintain a constant temperature of the sensor element 12,the amount of electric power application (electric power) to the heaterwas subjected to the feedback control depending on the change of theenvironmental temperature. Therefore, the difference was measuredbetween the amount of electric power application (electric power) to theheater without wind and the amount of electric power application(electric power) to the heater brought about when the fluid flowedthrough the exhaust tube 54 at a flow rate of 45 m/sec, and thedifference was used as a result of the measurement of the temperaturechange.

The measurement was performed when the gas sensor 10 was attachedperpendicularly to the exhaust tube 54 (hereinafter referred to as“standard attachment”) as shown in FIG. 4 and when the gas sensor 10 wasattached while being inclined (angle of inclination θ=35°) to theexhaust tube 54 (hereinafter referred to as “inclined attachment”) asshown in FIG. 5.

Comparative Example 1 was prepared such that the ratio A1/A2 between thetotal opening area Al of the inner gas inlet holes 22 and the totalopening area A2 of the outer gas inlet holes 34 was less than 1.

Example 1 was constructed such that the ratio A1/A2 was not less than 1,and the diameter of the outer gas inlet holes 34 was smaller than thatof Comparative Example 1. Example 2 was prepared such that the ratioA1/A2 was not less than 1, and the number of the inner gas inlet holes22 was larger than that of Comparative Example 1. Example 3 was preparedin the same manner as the gas sensor 10A according to the firstembodiment described above, in which the ratio A1/A2 was not less than1, the diameter of the outer gas inlet holes 34 was smaller than that ofComparative Example 1, and the number of the inner gas inlet holes 22was larger than that of Comparative Example 1.

FIG. 6 shows results of the measurement of wetting, and FIG. 7 showsresults of the measurement of the temperature change of the sensorelement 12. In FIG. 7, left bars indicate the cases of the standardattachment, and right bars indicate the cases of the inclinedattachment.

FIG. 6 shows the result of measurement of wetting. In the case of thestandard attachment, four out of twenty samples were NG (i.e., no good)in Comparative Example 1, two out of twenty-two samples were NG inExample 2, but no NG sample was found in both of Example 1 and Example3.

In the case of the inclined attachment, fifteen out of twenty sampleswere NG in Comparative Example 1, three out of twenty samples were NG inExample 1, fourteen out of twenty samples were NG in Example 2, and oneout of twenty-one samples was NG in Example 3.

As described above, in Comparative Example 1 and Example 2, thefrequency of the occurrence of NG differed depending on the attachmentstate of the gas sensor 10. In particular, the occurrence of NG wasconspicuous in the case of the inclined attachment. However, the numberof NG samples was smaller in Example 2 than in Comparative Example 1.

On the other hand, the wetting was scarcely caused in Examples 1 and 3irrelevant to the attachment state.

The temperature change of the sensor element 12 was observed as followsas shown in FIG. 7. In the case of the standard attachment, thedifferences in amount of electric power application were about 2.1 W inComparative Example 1, about 2.05 W in Example 1, about 1.2 W in Example2, and about 1.8 W in Example 3.

In the case of the inclined attachment, the differences in amount ofelectric power application were about 2.15 W in Comparative Example 1,about 1.5 W in Example 1, about 2.1 W in Example 2, and about 1.4 W inExample 3.

As described above, it is understood that, in Comparative Example 1, thedifference in amount of electric power application is large and thetemperature change is large irrelevant to the attachment state.

On the other hand, in Example 1, the temperature change is large in thestandard attachment state in approximately the same manner as inComparative Example, 1, but the difference in amount of electric powerapplication is small in the inclined attachment state. In Example 2, thetemperature change is large in the inclined attachment state inapproximately the same manner as in Comparative Example 1, but thedifference in amount of electric power application is small and thetemperature change is small in the standard attachment state.

In Example 3, it is appreciated that the difference in amount ofelectric power application is small as compared with Comparative Example1 in both of the standard attachment state and the inclined attachmentstate, and the temperature change of the sensor element 12 is suppressedregardless of the attachment state.

Next, in the second exemplary experiment, an observation was made forthe response performance when the A/F (air-fuel ratio) was changed forComparative Example 1 and Examples 1 to 3 described above.

An unillustrated 1.8-liter gasoline engine was used. Under a conditionof 2,500 rpm/26 Nm and a gas temperature=380°, the gas sensor 10 wasattached perpendicularly to the exhaust tube 54 as shown in FIG. 4. Asshown in FIG. 8, a response time T1 was measured. The response time T1ranged from a time point t11 at which the gas sensor 10 detectedA/F=13.2 to a time point t12 at which the gas sensor 10 detected A/F=21when the fuel was changed from rich to lean. Further, as shown in FIG.9, a response time T2 was measured. The response time T2 ranged from atime point t13 at which the gas sensor 10 detected A/F=21 to a timepoint t14 at Which the gas sensor 10 detected A/F=13.2 when the fuel waschanged from lean to rich.

Further, as shown in FIG. 10, two gas sensors 10 a, 10 b were attachedperpendicularly to the exhaust tube 54 so that the gas sensors 10 a, 10b were opposed to one another. As shown in FIG. 11, a delay time T3 wasmeasured. The delay time T3 ranged from a time point t15 at which onegas sensor 10 a (Comparative Example 1) detected A/F=17.1 to a timepoint t16 at which the other gas sensor 10 b (Example 1, 2, or 3)detected A/F=17.1 when the fuel was changed from rich to lean. Further,as shown in FIG. 12, a delay time T4 was measured. The delay time T4ranged from a time point t17 at which one gas sensor 10 a (ComparativeExample 1) detected A/F=17.1 to a time point t18 at which the other gassensor 10 b (Example 1, 2, or 3) detected A/F=17.1 when the fuel waschanged from lean to rich.

FIG. 13 shows results of the measurement of the response times T1, T2 inComparative Example 1 and Examples 1 to 3, and FIG. 14 shows the delaytimes T3, T4 in Examples 1 to 3.

According to FIGS. 13 and 14, the following fact is appreciated. Example1 involves the delay of a degree of about 30 msec as compared withComparative Example 1,but the response time T1 in Example 1 is shorterthan that in Comparative Example 1. Example 2 involves the delay of adegree of about 70 to 80 msec as compared with Comparative Example 1,and the response time T1 in Example 2 is longer than that in ComparativeExample 1 by about 10 msec. Example 3 involves the delay of a degree ofabout 40 msec to 60 msec as compared with Comparative Example 1, but theresponse time T1 is scarcely changed between Comparative Example 1 andExample 3.

Next, in the third exemplary experiment, an observation was made for theresponse performance obtained when the NOx concentration in a combustiongas of propane was changed for Comparative Example 1 and Examples 1 to 3described above. The gas temperature was 380° C., and the flow rate was10 m/s.

As shown in FIG. 4, the gas sensor 10 was attached perpendicularly tothe exhaust tube 54. As shown in FIG. 15, a response time T5 wasmeasured. The response time T5 ranged from a time point t19 at which thegas sensor 10 detected the NOx concentration=33% to a time point t20 atwhich the gas sensor 10 detected the NOx concentration=66% when the NOxconcentration in the gas was changed from low concentration to highconcentration. Further, as shown in FIG. 16, a response time T6 wasmeasured. The response time T6 ranged from a time point t21 at which thegas sensor 10 detected the NOx concentration=66% to a time point t22 atwhich the gas sensor 10 detected the NOx concentration=33% when the NOxconcentration in the gas was changed from high concentration to lowconcentration.

FIG. 17 shows results of the measurement of the response times T5, T6for Comparative Example 1 and Examples 1 to 3. Both of the responsetimes T5, T6 of Comparative Example 1 were about 750 msec. Both of theresponse times T5, T6 of Example 1 were about 790 msec. Both of theresponse times T5, T6 of Example 2 were about 800 msec. Both of theresponse times T5, T6 of Example 3 were about 700 msec.

As described above, it is appreciated that Comparative Example 1 andExamples 1 to 3 had the approximately identical response times.

As described above, in Examples 1 to 3, the ratio A1/A2 between thetotal opening area Al of the inner gas inlet holes 22 and the totalopening area A2 of the outer gas inlet holes 34 is not less than 1.Therefore, the flow rate of the measurement gas to flow into the innergas inlet holes 22 may be low, and the response performance of thesensor element 12 may be deteriorated. However, as appreciated from thesecond exemplary experiment and the third exemplary experiment, it ispossible to suppress the deterioration of the response performance byappropriately selecting the diameters of the outer gas inlet holes 34and the diameters of the inner gas inlet holes 22.

Next, a gas sensor 10B according to a second embodiment will beexplained with reference to FIG. 18.

The gas sensor 10B according to the second embodiment is configured inapproximately the same manner as the gas sensor 10A according to thefirst embodiment described above. However, as shown in FIG. 18, the gassensor 10B is different from the gas sensor 10A in that plate sections40 are provided for the inner protective cover 16 to extend overrectangular inner gas inlet holes 22, and the number of the inner gasinlet holes 22 is six.

Further, the gas sensor 10B is also different from the gas sensor 10A inthat the flange section 28 of the intermediate protective cover 20 ispositioned lower than that in the first embodiment, and the positionalrelationship of the outer gas inlet holes 34, the slits 32, and theinner gas inlet holes 22 is established such that the outer gas inletholes 34, the slits 32, and the inner gas inlet holes 22 are arranged inthis order from the bottom to an upper part of the outer protectivecover 18.

As shown in FIG. 19, each of the plate sections 40 includes two sidewalls 40 a, 40 b which rise toward the center of the inner protectivecover 16 from opposing circumferential portions of the inner gas inlethole 22, and a flat plate section 40 c which connects the side walls 40a, 40 b integrally and which is in parallel to the opening of the innergas inlet hole 22. In other words, the measurement gas passes throughthe portions other than the two side walls 40 a, 40 b and the flat platesection 40 c.

Therefore, the flat plate section 40 c gets the flow of the measurementgas coming into the inner gas inlet hole 22, and the measurement gas isdiffused. The measurement gas flows through the inner gas inlet hole 22,and the measurement gas is transmitted to the sensor element 12 disposedin the inner protective cover 16. As described above, the sensor element12 is prevented from any blow of the measurement gas, and it is possibleto suppress the temperature change in the sensor element 12.

Specific dimensions of the protective cover 14 of the gas sensor 10Baccording to the second embodiment may be described as follows by way ofexample. That is, as for the outer protective cover 18, the outerdiameter Do of the outer protective cover 18 is about 14.6 mm, thediameter d1 of the outer gas inlet hole 34 is about 2 mm, and thethickness of the outer protective cover 18 is about 0.4 mm.

As for the inner protective cover 16, the outer diameter Di is about 10mm, the long side d2 of the inner gas inlet holes 22 is about 3 mm, andthe thickness of the inner protective cover 16 is about 0.3 mm. Thediameter d3 of the intermediate gas discharge hole 26 provided at thebottom of the intermediate protective cover 20 is about 1 mm.

The distance L11, which ranges from the flange section 28 to the centerof the inner gas inlet hole 22, is about 6.3 mm. The distance L12, whichranges from the end of the outer protective cover 18 to the center ofthe outer gas inlet hole 34, is about 3 mm. The distance L13, whichranges from the center of the outer gas inlet holes 34 to the flangesection 28, is about 7.5 mm. The distance L14, which ranges from the endof the outer protective cover 18 to the end of the intermediateprotective cover 20, is about 4 mm.

It is a matter of course that the gas sensor according to the presentinvention is not limited to the embodiments described above, which maybe embodied in other various forms without deviating from the gist oressential characteristics of the present invention.

As described above, according to the gas sensor concerning the presentinvention, it is possible to effectively reduce the droplets of waterand the temperature change of the sensor element which would beotherwise caused by the inflow of the measurement gas, withoutdeteriorating the response performance, and the temperaturecharacteristics and the water resistance are excellent.

1. A gas sensor comprising a sensor element which measures apredetermined gas component of an introduced measurement gas, and aprotective cover which surrounds said sensor element, said protectivecover including: an inner protective cover which covers at least an endportion of said sensor element, an outer protective cover which coverssaid inner protective cover, and an intermediate protective cover whichis installed between said inner protective cover and said outerprotective cover, wherein said inner protective cover has abottom-equipped cylindrical shape with a plurality of inner gas inletholes which are formed at positions of a side surface thereof facingsaid sensor element and with at least one inner gas discharge hole whichis formed at a bottom portion; said outer protective cover has abottom-equipped cylindrical shape with a plurality of outer gas inletholes, said outer gas inlet holes formed in a side surface of said outerprotective cover at portions where said outer gas inlet holes do notface said inner gas inlet holes; said intermediate protective cover hasat least intermediate gas inlet holes which are formed at positionswhere said intermediate gas inlet holes do not face said inner gas inletholes and said outer gas inlet holes; andA 1/A 2≧1 provided that A1 represents a total opening area of said innergas inlet holes, and A2 represents a total opening area of said outergas inlet holes.
 2. The gas sensor according to claim 1, wherein thenumber of said inner gas inlet holes is larger than the number of saidouter gas inlet holes.
 3. The gas sensor according to claim 1, whereinsaid inner protective cover has plate sections each of which extendsover each of said inner gas inlet holes.
 4. The gas sensor according toclaim 1, wherein said inner gas inlet holes are formed at approximatelyequal pitches along one circumference of said inner protective cover. 5.The gas sensor according to claim 1, wherein said inner gas inlet holesare classified into first, second, . . . and nth groups; inner gas inletholes in said first group are formed at approximately equal pitchesalong a first circumference of said inner protective cover; inner gasinlet holes in said second group are formed at approximately equalpitches along a second circumference of said inner protective cover; andinner gas inlet holes in said nth group are formed at approximatelyequal pitches along an nth circumference of said inner protective cover.6. The gas sensor according to claim 1, wherein said protective cover isattached substantially perpendicularly to a gas tube.
 7. The gas sensoraccording to claim 1, wherein said protective cover is attached whilebeing inclined to a gas tube.